development of intelligent power unit for hybrid … · development of intelligent power unit for...

International Journal of Automotive Technology, Vol. 4, No. 2, pp. 57−64 (2003) Copyright © 2003 KSAE1229−9138/2003/012−01

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DEVELOPMENT OF INTELLIGENT POWER UNIT FOR HYBRID FOUR-DOOR SEDAN

K. AITAKA *, M. HOSODA and T. NOMURA

Hona R&D Co., Ltd., Engineering Development Department H1, 4630 Shinotakanezawa,Haga-machi, Haga-gun, Tochigi 321-3393, Japan

(Received 10 October 2002; Revised 21 February 2003)

ABSTRACT −−−−The Intelligent Power Unit (IPU) utilized in Honda’s Civic Hybrid Integrated Motor Assist (IMA) systemwas developed with the aim of making every component lighter, more compact and more efficient than those in the formmodel. To reduce energy loss, inverter efficiency was increased by fine patterning of the Insulated Gate Bipolar Transis(IGBT) chips, 12V DC-DC converter efficiency was increased by utilizing soft-switching, and the internal resistance othe IMA battery was lowered by modifying the electrodes and the current collecting structure. These improvemenreduced the amount of heat generated by the unit components and made it possible to combine the previously sepaPower Control Unit (PCU) and battery cooling systems into a single system. Consolidation of these two cooling circuinto one has reduced the volume of the newly developed IPU by 42% compared to the former model.

KEY WORDS : Battery, Cooling, DC-DC, Efficiency, HEV (hybrid electric vehicle), Inverter

1. INTRODUCTION

In recent years, the reduction of CO2 emissions throughincreased automotive fuel economy has become asignificant issue for the automotive industry. One solutionto this has been an increased focus on hybrid vehiclesystems combining internal combustion engines withelectric motors. Honda itself developed and marketed amodel of the Insight equipped with a lightweight andcompact Integrated Motor Assist (IMA) system in 1999.This paper discusses the development of the componentsof the Intelligent Power Unit (IPU) utilized in the IMAsystem for the 2001 Civic Hybrid. With the fundamentalaim of increasing the popularity of hybrid cars, thisdevelopment succeeded in producing a lightweight andcompact unit capable of being installed in a four-doorsedan.

2. DEVELOPMENT GOALS FOR THE CIVIC HYBRID IPU

The IPU utilized in the Civic four-door sedan wasdeveloped to allow the IMA system to be installed in aconventional sedan with the amenity of passenger roomand trunk space. The basic concept was to ensure cabin

space and associated trunk space for five people. Tnecessitated a reduction in the volume of the IPU by 4over the Insight unit to place behind the rear seat.

2.1. Reduction of the Size of the IPUThe IPU utilized in the Insight is shown in Figure 1. Thvolume of this former IPU was 138L, made up of:(1) Power Control Unit (PCU) 33L (24%);(2) Battery 43L (31%);(3) Cooling units 62L (45%).The target volume for the Civic Hybrid IPU is set as 80with the following make up of:(1) PCU 20%;

*Corresponding author. e-mail: [email protected] Figure 1. Dimensions of insight IPU.

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58 K. AITAKA, M. HOSODA and T. NOMURA

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(2) Battery 30%;(3) Cooling units 60%.

2.2. Integration of Cooling UnitsParticular attention was focused on reducing the size ofthe cooling system by integrating the twin cooling circuitsin the former IPU (battery and PCU cooling units). Givingconsideration to the management of the appropriatebattery temperature, it is necessary for the intake air,which functions as the battery cooling medium, to be atthe normal temperature. It was therefore established onthe assumption that intake and exhaust of cooling airwould be conducted within a vehicle. Because internalintake and exhaust would increase the load for air-conditioning and cause noise in the cabin, target airflowvolume was established as less than 90 m3/hr. The airflowrequired by the Insight cooling system is:(1) Inverter 138 m3/hr;(2) 12V DC-DC converter 120 m3/hr;(3) Battery 60 m3/hr.

For integration of the cooling systems, it is necessary toreduce the airflow requirement of the inverter and the12V DC-DC converter to below 90 m3/hr. In the formerPCU cooling air was taken in from outside the vehicle,meaning that it was necessary to set the intake temperaturehigh under the situation of hot summer weather. Butintegration of the cooling systems in the new modelwould enable internal air circulation and therefore lowerair temperatures, leading to a reduction of the airflowrequired. In combination with this, it would be necessaryto reduce the heat generated by the PCU by increasing theefficiency of each component. In this way, the developmentprocess examined the possibility of integration of theformer model’s two cooling systems by reduction of thetemperature of the intake air and increased efficiency incomponents.

2.3. Responding to Increased Assist and RegenerativePowerCivic Hybrid was heavier than the Insight, and was alsoadopted a Cylinder Idling System (CIS), meaning that theIPU had to respond to a 30% increase in assist andregenerated power. This also required increased efficiencyin all the IPU components, and formed an importantaspect of the development agenda for this project.Increased heat generation caused by increased input andoutput power to battery was a particular consideration. Torespond to this, it was necessary to increase the batterycooling airflow 50% and decreasing internal resistance inthe battery 20% over the former model to decrease theamount of heat generated, while maintaining the targetfor maximum airflow in the integrated cooling systemmentioned in the previous section.

3. POWER CONTROL UNIT (PCU)

To position the IPU behind the rear seat, it was necessto make the PCU approximately 20% smaller and reduenergy loss in the inverter and the 12V DC-DC convertThis was achieved in the following way:(1) Integration of the inverter and the pre-driver unit;(2) Fine patterning of the Insulated Gate BipolaTransistor (IGBT) chips;

(3) Reduction of energy loss and the adoption ofstructure enabling high heat dissipation in the capacit

(4) Adopting a soft switching method for 12V DC-DCconverter control.

These developments enabled a 20% reduction in volume of the newly developed PCU, which is 26.8against the 33L for the Insight PCU. Figure 2 shows tPCU utilized in the Civic Hybrid.

3.1. InverterTo make the unit smaller and lighter and reduce eneloss, new technologies were developed for the comnents making up the inverter. This section providdetails of these technologies.

3.1.1. Integration of inverter and pre-driver unitThe improvements of inverter which controls the motdrive are as follows:

(1) Custom integration of IGBT element drive circuits(2) Installation of a high-performance microcompute

to provide the software processing as a safegufunction;

(3) Simplification of interface circuits by adoption oserial communication between Motor ECU, etc..

These new technologies have enabled a 39% reducin size and a 28% reduction in weight over the form

Figure 2. Power control unit.

DEVELOPMENT OF INTELLIGENT POWER UNIT FOR HYBRID FOUR-DOOR SEDAN 59

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model. Figure 3 shows the inverter unit utilized in theInsight and the Civic Hybrid. In the former PCU, over-heating was prevented by heat-detecting thermistorspositioned near the IGBT chips. The responsiveness oftemperature detection in the new model has been improvedby locating temperature sensors above the IGBT chips.

3.1.2. Reduction of loss by fine patterning of IGBT chipsFigure 4 shows the cell pattern of the IGBT chip, thepower element in the inverter. Refinement of the cellpattern compared to the former model has enabledapproximately three times more cells to be mounted onthe same surface area and reduction of channel resistanceper unit of surface area (Rch). Therefore, ON voltage forthe IGBT has been lowered and loss has been reduced amaximum of 25% over the former model.

3.1.3. Reduction of capacitor sizeIn the former PCU, elements of the electrolytic capacitorwere fixed in resin for vibration resistance and heatdissipation. In the new model, heat dissipation occurs

through direct contact between the bottoms of telements and aluminum cases (Figure 5), improving h

Figure 3. Inverter unit.

Figure 4. Model of IGBT fine patterning.

Figure 5. Capacitor structure.

Figure 6. Inverter efficiency.

Figure 7. Inverter efficiency map.


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dissipation 20% over the former model. Improvement ofthe electrolyte and electrode foil etching has reducedresistance, leading to decrease of the Equivalent seriesresistor (ESR) and 18% less heat generation in the newelements. These improvements have enabled the capacitorto be reduced 30% in size and 27% in weight.

3.1.4. Increased inverter efficiencyThe technologies described above have reduced loss andincreased inverter efficiency under all motor driveconditions. Figure 6 shows a graph of inverter efficiencyagainst motor speed. A map of inverter efficiency plottedagainst motor speed and torque is shown in Figure 7.When motor speed and torque data sampled during actualdriving are plotted on this graph, it becomes clear thatduring actual driving the inverter’s high-efficiency rangeis mainly used. Reducted of energy loss by improvementof efficiency also contributes increase of fuel economy.

3.2. 12V DC-DC ConverterThe adoption of soft switching in the new 12V DC-DCconverter has reduced loss against the former hardswitching method. Figure 8 shows a conceptual diagramof operation of the circuit.

In switching operation of Metal Oxide SemiconductorField Effect Transistor (MOSFET) which drives atransformer, when the MOSFET are switched on, andcurrent from the IMA battery to the transformer via theMOSFET, switching timing is controlled by a resonantinductor in the current path, reducing the overlap timebetween the MOSFET drain-source voltage and the draincurrent, therefore reducing switching loss. As Figure 9shows, the adoption of this soft switching circuit hasimproved efficiency an average of 2% over the formermodel. This technology has also enabled internalsimplification of the converter, allowing a 22% size

reduction and a 9% weight reduction compared to tInsight model.

Figure 10 shows a map of efficiency of the 12V DCDC converter against input voltage from the IMA batteand output current from the 12V battery. Efficiency habeen improved in the areas utilized in actual driving shown in this graph, and loss has been reduced duactual use. Therefore, it become possible to cool the uwith a smaller airflow than required in the Insight, anthe consumption of electric power has also been reducThis has enabled the size of components to be reduand fuel economy to be improved.

4. IMA BATTERY

Compared to the Insight, the weight of the new vehichas increased, and it has also been provided withCylinder Idling System (CIS), increasing the energavailable from regeneration. It was predicted that tincrease of energy in assist and regeneration, that is,

Figure 8. 12V DC-DC converter circuit.

Figure 9. 12V DC-DC converter efficiency.

Figure 10. 12V DC-DC converter efficiency map.


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increase of input and output average power of the batteryduring driving, would increase the amount of heatgenerated by the battery. The following solutions wereexamined to respond to this:

(1) Approximately 20% reduction in internalresistance of the battery module;

(2) 50% increase in cooling airflow provided to batteagainst the former model.

Figure 11 shows the amount of heat generated in battery and the battery heat balance of the Insight andCivic Hybrid during actual driving. The Civic Hybridbattery generates 1.8 times as much heat as the Insbattery, but a 20% decrease in internal resistance an50% increase in the cooling airflow provided to thbattery have enabled temperature increase to be mained at the level of the former model.

4.1. Battery ModuleThe battery module was improved for exclusive use in Civic Hybrid. The electrodes and the current collectstructure were reexamined and the shape of the curcollecting plate in particular was altered to reduinternal resistance. Figure 13 shows a comparison of shapes of the current collecting plate in the former anew models.

The change in the shape of the current collecting plenabled the amount of welding points with the electrod

Figure 11. Battery heat balance.

Figure 12. IMA battery module.

Figure 13. Current collecting plate.

Figure 14. IMA battery box.

Figure 15. Cooling airflow in Insight IPU.


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to be increased, and standardized the current density onthe plate. In combination with internal improvements tothe battery, resistance has been decreased 19% over theInsight model.

4.2. IMA Battery BoxBattery box parts were also re-examined to reduce theweight and size.

(1) The placement of the components attached to thebattery box (battery ECU, harness, etc.) was optimized,enabling reduction of the size of the box unit.

(2) The intensity of the box was increased, and its sizeand weight were reduced by the design of couplingsoptimized for the Civic Hybrid.

As a result, the volume of the new box has beenreduced by 29.5% and the weight by 5.6% compared tothe box used in the Insight.

5. INTEGRATION OF COOLING SYSTEMS

Figure 15 shows the Insight IPU and its cooling circuit. Inthis model there are separate cooling circuits for the PCUand the battery. The PCU cooling system takes in airfrom outside the cabin, and exhausts the air outside thecabin after cooling the inverter and the 12V DC-DCconverter. The battery cooling system takes in air fromthe cabin, and exhausts the air back into the cabin aftercooling the battery. These cooling systems make upapproximately 45% of the volume of the entire IPU.Integration of the cooling systems was examined as amethod of reducing the size of the cooling system in theCivic Hybrid by 60%.

Figure 16 shows the Civic Hybrid IPU and its coolingcircuit. The Civic Hybrid IPU is positioned behind therear seat. Cooling air for the battery is taken in throughthe inlet located above the rear tray inside the vehicle.After cooling the battery, the air is reused to cool the

PCU, and exhausted into the trunk. A section of the IPcase is used as a cooling circuit from the battery exhato the PCU inlet instead of a duct, and it enablreduction of the number of components. The main poiof difficulty encountered with this cooling system are afollows:

(1) Establishment of PCU cooling system by battecooling air exhaust;

(2) Suppression of cabin noise caused by IPU coolair;

(3) Responding to increase in battery temperature hot days caused by vertical placement of battery bbehind rear seat.

The results of studies on (1) will be discussed here; and (3) will be discussed in the following sections.

5.1. Study of Temperature of IPU Cooling AirBecause cooling air for the PCU was drawn from outsithe vehicle in the Insight, the maximum temperature PCU cooling air intake was set at 60 deg.C, consideration of operation during hot summer weathetc. For the Civic Hybrid, to lower the intake temperatuof PCU the cooling systems were integrated and the Pcooling system uses air exhausted from the battcooling system, which is drawn initially from within thecabin. Figure 17 shows the transition of the battery aPCU intake temperatures in the Civic Hybrid whe

Figure 16. Cooling airflow in Civic hybrid IPU.

Figure 17. Cooling air temperature when driving in htweather.

Figure 18. Characteristics of cooling inverter.


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driven after having been left outside on a hot day. Theintake temperature above the rear tray (battery intaketemperature) falls with the commencement of operationand when the vehicle is cruising reaches around 35deg.C. The air reaches about 50 deg.C from cooling thebattery, and is used to cool the PCU. This fact was thebasis of a study of the possibility of establishing a coolingsystem with a maximum temperature of 50 deg.C.

5.2. Study of Feasibility of Reduction of Cooling AirflowIt was possible to reduce the maximum temperature ofthe air intake to 50 deg.C, 10 deg.C lower than in theInsight, by using battery cooling air to cool the PCU. Astudy was conducted to determine the relative reductionof necessary cooling airflow granted by this method andthe previously mentioned reduction of loss in eachcomponent. As an example, Figure 18 shows therelationship between the temperature of the inverter andthe cooling airflow.

The reduction of loss by a maximum of 25% throughincreased inverter efficiency and the reduction of theintake air temperature has enabled cooling with anairflow of 60 m3/hr, 57% less than the previous model.Results for the 12V DC-DC converter similarly show thatcooling is possible with an airflow of 60 m3/hr, 50% lessthan required in the previous model. Despite increasedefficiency through reduction of internal resistance in thebattery, increased vehicle weight and increased regener-ative energy have meant a higher level of battery inputand output power, necessitating 90 m3/hr of cooling air,50% more than required in the previous model. Figure.19 shows the results of the reduction of cooling airflow.As this graph shows, the maximum cooling air require-ments for the battery, inverter and 12V DC-DC converterare all below the target maximum of 90 m3/hr. As a result,it has been possible to cool all components within thesystem at an airflow rate less than a maximum establish-ed on the basis of consideration of increased load for airconditioning and in-cabin noise caused by internal intakeand exhaust of cooling air.

Reduction of the airflow required to cool each

component to within the target and reduction of the sof the components themselves has enabled integratiothe cooling circuits. The number of cooling fans anducts has also been reduced, and the amount of volumthe IPU taken up by the cooling system has bedecreased 62% against the former model.

6. REDUCTION OF COOLING FAN NOISE

In addition to integration of the IMA cooling circuits, aPWM control type IPU cooling fan was newly developefor the Civic Hybrid. This new fan has realized step-lefine control.

Increasing airflow is a simple method to improvcooling ability, but the higher intake of air required fromthe cabin increases noise, with a consequent negaeffect on the market appeal of the vehicle. A new fcontrol method was therefore developed for the CivHybrid in order to achieve a high level of balancbetween cooling performance and product appeal. Tfan control duty factor was chosen as the larger of

Figure 19. Airflow rate.

Figure 20. IPU cooling fan.

Figure 21. Cooling fan noise.


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(1) The factor determined by the battery temperatureand the input and output current.

or(2) The factor determined by the difference of temper-

ature between each battery modules.To further reduce fan noise, a restriction was applied

depending on the upper limit for the duty factor deter-mined by vehicle speed. The target here was to minimizethe fan noise felt by passengers using the change of cabinnoise due to variations in vehicle speed, while maintain-ing cooling performance. The target for fan noise for theCivic Hybrid was established as 8dB less than the level ofin-cabin background noise. Figure 21 shows vehiclespeed and noise level, and Figure 22 shows a graph of theupper limit of duty for fan operation.

The adoption of the fan control described above hasmade possible a silent cooling system which stillprovides the required cooling performance.

7. PREVENTION OF BATTERY MODULE HEATING

Positioning of the IPU behind the rear seat has necessitat-ed vertical placement of the battery box, meaning thatwhen the vehicle is left out on hot days the batterymodule, which is located in the upper section of the box,can become overheated in the sun. In the Civic Hybrid,heat insulators have been placed around the battery boxto prevent this phenomenon.

The main insulation measures are:(1) Placement of insulators around the battery box;(2) Manufacture of the intake duct out of forming resin

to give it an heat insulating function. It is also providedwith a rubber heat isolation valve.

As a result, the battery temperature rises approxima10 deg.C less when the vehicle is left out in the sueffectively preventing overheating. Because expandapolypropylene beads with excellent impact absorptiproperties are used as the heat insulator, it doublesprotect the battery box in the event of a collision.

8. CONCLUSION

New technologies have been utilized in each componeenabling reduced generation of heat through increaefficiency, and each component to be made smaller.particular, increased efficiency in the inverter and the 1DC-DC converter has reduced the amount of heat gerated despite the increased electrical load. Simplificatof the overall IPU cooling system has also been possibwith the two cooling systems of the Insight IMA systembeing integrated into one. As a result, the volume of tnewly developed IPU is 42% lower than the Insigmodel. This means that the IMA system can be instalbehind the rear seat in the trunk while maintaining tsame passenger space as a conventional sedan andminimal loss of trunk space.

REFERENCES

Matsuki, M., Wakashiro, T., Kamiyama, T., Sato, TKaku, T. and Kanda, M. (2002). Development of ANew Power Train for the Civic Hybrid, Honda R&DTechnical Review 14, 1, April, Japan.

Figure 22. Cooling fan control.

Figure 23. Heat insulating device for IMA battery.

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